Traditional composite structures made by multidirectional laminated construction techniques suffer from two major deficiencies: 1) performance limits imposed by micro-cracking and delamination failure modes; and 2) high cost of manufacturing resulting from the lack of automation and modular construction. The most severe limit imposed by the traditional design is the low allowable strain of 0.4 percent. Micro-cracking of the matrix is responsible for this low limit. Delamination failure imposes another limit that often controls the laminate design.
The most efficient use of composite materials is to use them in their unidirectional format. This is done in filament wound pressure vessels where loading is tensile only. For more general states of stress, laminated composite are required to sustain multidirectional loading conditions. The strength potential, however, is then severely limited because of microcracking and delamination.
Grid structures avoid many of the problems of traditional composite structures. All ribs are unidirectional, and are not susceptible to micro cracking and delamination. One of the most common methods for constructing composite grid structures uses a soft rubber tooling which has been pioneered by USAF Phillips Laboratory. This process has been used to manufacture a solar panel substrate, missile fairing and adapters. Soft tooling, however, does not provide good finish nor does it maintain constant rib thickness. The soft tooling must be peeled off after curing, which is difficult if not impossible without causing damage to the tooling and/or the part. For this reason, the depth of the rib is limited to 2.5 cm for a grid size of 12.5 cm. Higher ribs would make the tool removal nearly impossible.
Another accepted practice of grid manufacturing is to cut grooves in a rigid foam. Similar to the soft tooling method, fiber bundles and a resin binder can then be forced into the grooves to form a grid structure. The foam can in fact be left in the grid to provide the sound and thermal barrier if that is required. The most severe limitation of this process is at the intersection points in the grid structure where the fiber volume fraction is limited to 60 percent. This in turn places a limit on the fiber volume in the ribs. In fact, the ribs would have only one half of that value; i.e., 30 percent. At this low fiber volume, the performance of the grid structure would not be competitive with laminated structures of the same composite material. Present grid structures have the additional disadvantage that they are still hand-made, resulting in expensive and slow production.
Other processes that use hard tooling, such as metals, suffer from the same deficiencies, such as low fiber volume fraction, limited height, and intensive labor required. Many molded composite gratings fall into this category. Others use laminates wrapped around metallic cores and subsequently consolidated to form "egg-crate" grids. This type of manufacturing is labor intensive and not practical for larger structures.
The prior art process of using rubber tooling has not met all the performance and cost objectives. Tooling continues to be a challenge. Rubber tooling often breaks and cannot be reused. Control in fiber packing and geometry of the ribs is also difficult. The most important limitation is the fiber volume fraction in the rib that is controlled by the fraction at the nodal points. Even with a nodal offset, the fraction at the rib is essentially one half of that at the node.
Slotted joint grids (FIG. 6) are often found in carpentry and have been applied to composite grids. Difficulties include the optimum tolerance of the cut slots and the bonding to seal the joints. Substantial stress concentrations at the cut-out edges also impair their strength. Other designs use laminated composites for the ribs. The effective stiffness of laminates is less than one half of that of the unidirectional ribs.